Every year from the 1990's has something that I really enjoy, but specifically:
1991 - for all the death metal everyone's talking about
1994 - for all the black metal people have been talking about
1994 - for some really good mainstream releases from Soundgarden, Stone Temple Pilots, Nine Inch Nails.
1999 - because I loved almost everything about that year (May + October thru December sucked)

The first half of the decade (90-94) was very plentyful. I'm gonna go 90' though as with the exception of black metal, it had a great balance of everything (but even with black metal we got Live In Liepzig).